In recent years, a client server system employing a personal computer (PC) has come into wide use due to down-sizing. In the client server system, a server executes the database retrieval for example in response to requests from a plurality of clients, and hence a high data processing capability is required therefor. For this reason, there has developed a construction which employs an operating system (OS) for a multiprocessor and in which a plurality of processors are mounted on a PC used as a server (hereinafter, the PC of this sort is particularly referred to as "a PC server" for short, when applicable) to construct an operating system and the processings are distributed among these processors.
As for a system for mounting a plurality of processors on one PC, there is known a system employing a processor board. Conventionally, this system is a system which has been used in a relatively large scale workstation or personal computer. This system is such that a board (a processor board) on which one or two processors and a cache system are mounted is connected through a connector to a system bus which is distributed on a main board and which is called a shared bus in order to be used. On the main board, there are disposed a memory interface, a memory module, an I/O bus bridge, an I/O interface board and the like. The details of this system are described in an article of "MULTIPROCESSOR SYSTEM ARCHITECTURE", p. 269, published by PRENTICE HALL Corporation.
In the system employing the processor board as described above, an interface LSI for a connector and a shared bus is required every processor board. In addition, in order to enhance the performance of the system, the cache system and the clock generator which is locally arranged are mounted on the processor board. As a result, the number of components of the overall system is increased. In addition, since the high speed shared bus signals are connected through the connector, the price of the system becomes relatively high due to the factors such as the use of the connector through which the signal is transmitted with less distortion and which is excellent in electrical characteristics and the complicated chassis structure resulting from the fixing of a plurality of boards.
For this reason, in recent years, as the integration of the LSIs has been increased, the processor which self-contains the bus interface and the cache for use in constructing the multiprocessor is also on the market. Then, a plurality of processors, the memories-and the I/O interface are disposed on one main board by employing such a processor.
Now, the interchangeability of the I/O interfaces is very important in the PC server. The I/O interfaces for the PC are on the market in the form of the expansion boards, and hence a user may select and purchase desired ones on the basis of the performance and the introduction cost which are required for the server system. In addition, it is also widely carried out that a user himself/herself attaches and detach the expansion cards thereto and therefrom. In such a way, if the I/O interfaces are made common and also the operating hardware is not limited, this assists the free price competition among the I/O expansion board makers, and also a user can always construct the system which is excellent in price-to-performance ratio by employing the newest and high performance products.
Currently, in the PC server, the I/O bus specification called a PCI is widely used. The PCI bus is the local bus for which the standardization is in progress under the leadership of INTEL CO. LTD., U.S.A., by the standardization group i.e., the PCI Special Interest Group, and the expansion boards each conforming to this specification have the common size, connection connector shape and bus protocol. The details thereof are described in an article of "PCI LOCAL BUS SPECIFICATION REV. 2. 1, Jun. 1, 1995" published by the above-mentioned Group. As for the PCI expansion cards, the card having the board size called LONG CARD, the card having the board size called SHORT CARD, and the card having the intermediate board size are on the market. Since a large number of expansion boards, such as the high speed network I/F and the desk array I/F, each of which is excellent in I/O processing performance are of a LONG CARD type, preferably, the PC server can use the expansion card of LONG CARD if possible. This PCI bus is the unterminated bus which is operated at a frequency of 33.3 . . . MHz at maximum, and hence in order to use the bus and the I/O interface at the highest frequency, it is necessary to take the length of the transmission line of the PCI bus and the electrical characteristics thereof into consideration.
In order that a plurality of processors may be disposed on the same main board and also the PC server may be designed which is excellent in price-to-performance ratio, the various problems need to be solved. In recent years, a large number of processors each of which is operated at the frequency equal to or higher than 100 MHz are on the market. In particular, in the case of the high end processor which is used in the PC server, the power consumption thereof is large on the order of about 10 to 30 W. For this reason, in order to cool the processor, it is essential to the processor of this sort to attach the heat sink thereto. While the heat radiation characteristics of the heat sink are varied depending on the temperature within the chassis and the speed of air flowing through the fins of the heat sink, in general, in order to cool the heat radiation of about 30 W, it is necessary to employ the heat sink which has the component height equal to or larger than 30 mm. This component height is larger than the minimum distance (a clearance) between the base of the board in which the card edge is disposed and the main board in the PCI expansion card. Therefore, there arises the problem that the PCI expansion card may come in contact with the heat sink depending on the positions of the processors so that the printed circuit on the PCI expansion card will be short-circuited and the PCI expansion card of LONG CARD will not be able to be plugged into the PCI expansion slots.
On the other hand, the price-to-performance ratio is important in the PC server, and hence it becomes important that the area of the main board is reduced as much as possible to reduce the manufacturing cost of the board and the chassis. For this reason, how the PCI expansion card, the connector and the processors are disposed without interference becomes the important design standard. In addition, ensuring the flow of air flowing through the heat sink and how the calorification from the processors is effectively cooled are the problems in the chassis design.
In general, since a plurality of processors and LSIs are connected to the processor bus, the terminated bus is employed which is capable of realizing the relatively long bus length for the same clock as a result of suppressing the reflections at the end portions of the transmission line. On the other hand, each of the memory bus and the PCI bus is the unterminated bus, and hence in order to suppress the distortion of the signal waveform, it is desirable to distribute the transmission line length as short as possible. In addition, the PCI bus expansion cards such as the hard disk interface and the network interface which are connected to the PC server are now manufactured by various board makers. Then, though these cards are defined by the specification, the electrical characteristics thereof vary widely. In order that the PCI bus routed on the main board may be operated at 33.3 . . . Mhz as the maximum operating frequency of the PCI bus irrespective of the arbitrary combination of these cards, it is desirable to increase the margin to the timing requirements of the PCI bus as much as possible. Therefore, the trade off of the line length of the processor bus, the memory bus and the PCI bus is a serious problem.
In addition, since the high performance processor is operated at a frequency of a high speed clock, e,g., at 100 MHz, it is manufactured by utilizing the newest semiconductor process. Also, as for the package for accommodating the chip associated therewith, there is employed ones such as the Pin Grid Array (PGA) each of which is excellent in electrical characteristics. On the other hand, with respect to the LSIs such as the memory controller and the PCI bridge, it is desirable to reduce the price thereof by utilizing the general process. However, if the inexpensive process and package are employed, then the gate delay time elapsed till the electrical signal is outputted from the clock of the LSI, and the set up time become larger values than those of the associated processor, and hence the margin to the timing requirements becomes smaller all the more.